I have several bowstrings. I know some are fast flight and some are B-50. How do I tell the difference between them by just looking at them?
If you can get a tag end pulled loose (if they are flemish twist), the Dacron string will fairly easily cut with scissors, FF will not.
Allen is right. Also count the strands and check the dia. with one of you arrow nocks.
Also dacron, and high performance materials have a unique smell. Fresh dacron smells like crayola craons to me. Warm it up with a hairdryer, and take s sniff!
Some "FF" materials are small in diameter, but not all. Trying to clip a tag end with scissors or nail clippers works for me.